1. Field of Invention
The present invention relates generally to vacuum electronics and, more particularly, to the circuits and method for linearizing vacuum electronic amplification.
2. Description of the Background
Amplifiers come in many forms and are used in many applications. For example, amplifiers may be used with digital or analog signals, may be used in communications systems such as wireless telecommunications and satellite communications, and may be semiconductor based or vacuum tube based.
The performance demanded of amplifiers continues to increase, and many conventional amplifiers are failing to keep pace. For example, conventional semiconductor microwave amplifiers lack the power capabilities required by many modern microwave systems. As a result, vacuum tube power amplifiers are essential components of many modern microwave systems, including telecommunications, radar, electronic warfare, and navigation systems, because microwave tube amplifiers can provide microwave energy at levels of power higher by orders of magnitude in comparison to semiconductor microwave amplifiers. The higher power levels offered by tube devices are facilitated by the fact that electrons can travel at a much higher velocity in a vacuum than in a semiconductor. The higher velocity permits use of larger structures with the same transit time. Larger structures, in turn, permit greater power levels.
The power amplification for modern high power vacuum electronic microwave amplifiers (VEMAs), however, is typically non-linear. For instance, phase non-linearity may be caused when the electrons slow down while moving through an interactive region of the tube. That slowing is a result of the electrons losing kinetic energy as they amplify a signal passing through the tube. At high power levels, however, the electrons start to slow down significantly and desynchronize from the RF field in the interactive region, thereby causing a phase lag between the input and output signals. For example, without phase compensation, the phase delay of the output signal for a traveling wave tube (TWT) VEMA operating at its saturation point may be as great as 70.degree.-80.degree., which may be unacceptable for many applications, such as digital communications. Moreover, at high power levels, the beam current is not large enough to continue amplifying the input signal, causing an RF saturation or an amplitude non-linearity. Thus, non-linearity in power amplification generates higher-order intermodulation products, which may result in undesirable spectral regrowth in adjacent channels, and phase distortions, which in turn may increase bit errors in digital communications systems. The drawbacks are especially acute in digital wireless communications systems where multiple communication signals are typically multiplexed onto a single, narrow wavelength-band, channel due to the limited RF spectrum bandwidth. Accordingly, practical communication limitations as well as government standards require minimal higher-order intermodulation and phase distortion.
One technique to minimize power amplification non-linearity is to operate a VEMA only in its linear range, which typically is a small fraction of its power capacity. For example, for an application that requires 50 Watts of amplification power, a tube capable of 500 Watts of amplification power may be required to produce an adequate linear range. This technique, of course, is inefficient and expensive. Another known technique, commonly referred to as the feed forward technique, involves coupling the input signal to a low power linear amplifier in parallel to the main power amplifier to provide cancellation of the non-linear portions of the carrier signal at higher power levels. An additional method, referred to a the predistortion technique, involves distorting the characteristics of the input signal to the main power amplifier to cancel the non-linear characteristics of the amplifier. Both the feed forward technique and the predistortion technique, however, require the processing of the carrier signals, which for microwave applications is on the order of gigahertz. Consequently, both these techniques require very expensive circuitry which is difficult to tune.
Accordingly, there exists a need for an efficient and inexpensive technique for improving the linearity of high power vacuum electronic microwave amplifiers.